FIG. 1 is a simplified representation of a prior art spacecraft 10 in a geosynchronous orbit about Earth 12. Spacecraft 10 may be a communications satellite carrying a payload of transponders and antennas (not illustrated). As illustrated in FIG. 1, spacecraft 10 includes a body 18 associated with pitch, roll and yaw axes. In order to perform its communication or other function, spacecraft 10 must maintain its body 18 in a controlled orientation. The spacecraft may include an earth horizon sensor, illustrated as 28, and other sensors (not illustrated) for aiding in determining attitude, and may also include magnetic torquers (not illustrated) and/or reaction wheels or momentum wheels (not illustrated) to provide a measure of orientation or attitude control. A desired orientation might maintain, as illustrated, north face 20 of body 18 orthogonal to the pitch axis, east face 22 orthogonal to the roll axis, and the earth-facing side (not visible in FIG. 1) and anti-earth side 26 orthogonal to the yaw axis.
Ordinary orbital motions of the spacecraft tend to cause drift in a north-south direction away from the desired station. In the prior-art arrangement of FIG. 1, chemical thrusters designated 1, 2, 3 and 4 are located near the corners of north face 20. Other thrusters, representatively illustrated as thrusters 7 and 8 on east face 22, are also provided. Such chemical thrusters operate by expelling propellant mass heated by means of a chemical reaction. Such a chemical reaction may be based upon a bipropellant fuel-oxidizer system or upon a monopropellant-catalyst system, such as one which uses hydrazine propellant.
Also in FIG. 1, four electrothermal arcjet thrusters or "arcjets" designated 13, 14, 15 and 16 are located on north face 20, on either side of a conceptual bisecting line 24, which lies in a plane parallel to the pitch-roll plane (i.e., parallel to the plane which includes the pitch and roll axes). Arcjets such as 13-16 (where the hyphen represents the word "through") use the electrical energy of an arc to increase the enthalpy of propellant mass originating from a chemical reaction similar to that of the chemical thrusters. The energy of the electrical arc results in an increased specific impulse (I.sub.SP) of an arcjet over an equivalent chemical thruster. Specific impulse is a measure of the thrust resulting from a rate of propellant mass expulsion, with dimensions of lbf/lbm/sec=sec. An unaugmented hydrazine Rocket (or Reactive) Engine Assembly (REA) thruster operated in an on-pulsed mode has a specific thrust of about 150 seconds. An Electrothermal Hydrazine Thruster (EHT), which uses an electrical resistance heater to augment chemically derived propellant energy, has specific thrust in the range of 300 seconds, while an arcjet may reach 600 sec. The advantage of the arcjet is so great that it is used wherever possible, to the exclusion of other thrusters. U.S. patent application Ser. No. 07/285,699 filed Dec. 16, 1988 in the name of Davies et al now U.S. Pat. No. 5,263,666, describes advantages to exclusive use of monopropellant thrusters for attitude control and North-South stationkeeping. Stationkeeping maneuvers are performed by energizing a diagonally opposed pair of arcjets, such as the pair 13, 15 or the pair 14,16. This operation is performed around the ascending node of the orbit. Ideally, the arcjets of the pair would produce exactly the same amount of thrust, with the lines of action of the thrust mutually parallel, and at equal distances from the center of mass (not illustrated) of the spacecraft. With such an ideal condition, the station of the spacecraft could be corrected without affecting the attitude.
The various imbalances of the spacecraft and the thrust tend to result in a change in attitude as the stationkeeping arcjets are fired. These changes in attitude are sensed by the attitude control system (not separately illustrated), which energizes one or more of the monopropellant thrusters 1-4 on the north face, and possibly thrusters on other faces, to maintain correct attitude. The magnitude of the thrust of the desired attitude control thrusters is controlled by on- or off-pulsing, in known manner. The arcjets are not on-off pulsed, because they can be ignited only a limited number of times due to various factors such as cathode erosion and thermal stresses. It is desirable, when making the aforementioned attitude corrections, to use north-face thrusters 1-4 insofar as possible, to the exclusion of thrusters on other faces, because the attitude control thrust is in the direction of the desired stationkeeping thrust, and contributes to the desired velocity. Thrust on faces other than the north face does not so contribute. Thus, greatest propellant usage efficiency results, in the arrangement of FIG. 1, from operating arcjets 13-16 to obtain the desired stationkeeping thrust, and operating chemical thrusters 1-4 to maintain attitude during the stationkeeping maneuver.
It should be noted that, while the arcjets have high specific thrust, they do not necessarily have a large thrust, and in fact the arcjet thrust may be smaller than the thrust of a chemical thruster used therewith for attitude control. Therefore, the arcjets, once ignited may be operated for many minutes or even for hours, to achieve the desired velocity change.